The present invention relates to an underbody structure of a motor vehicle and, more particularly, to an underbody structure of the type having a floor panel which includes a honeycomb core.
Traditionally, the underbody of a motor vehicle is constituted by longitudinally extending side sills, laterally extending cross members and others which are adapted to give mechanical strength to a body of the vehicle, and a floor panel positioned between those members. usually implemented with a single flat member, the floor panel is connected at laterally opposite sides thereof to the side sills by spot welding.
A prerequisite with a motor vehicle is that the floor of a passenger compartment be as flat as possible. In this respect, a floor panel implemented with a single flat member has a drawback that increasing the area of the flat portion for the floor panel reduces the rigidity of the flat portion and thereby causes the flat portion to vibrate when the vehicle body vibrates. Further, a floor panel with poor rigidity is incapable of serving the other expected functions, i.e. , suppressing vibrations of the entire floor and intercepting noise otherwise transmitted to the passenger compartment.
In light of the above, it is a common practice to mount insulators or like sound insulating members on a floor panel together with cross members or like reinforcing members, so that sound insulation may be enhanced without affecting rigidity. However, such sound insulating members and reinforcing members not only increase the total weight of the vehicle body but also prevent the rigidity from being uniformly increased over the entire floor.
An implementation recently proposed to solve the above problems is replacing the conventional floor panel with a honeycomb panel which has a honeycomb core sandwiched between an upper and a lower plate and is extensively used with aircraft. Such a honeycomb panel is light weight and, yet, rigid while successfully damping vibrations and insulating sound. With a honeycomb panel, therefore, it is possible to realize a floor panel having an extremely wide flat portion inside of a passenger compartment.
A honeycomb panel stated above can hardly be shaped by pressing due to its considerable rigidity. Hence, when used for the floor panel of a motor vehicle, a honeycomb panel is applicable to flat portions only and not to a center tunnel portion and has to be rigidly connected to side sills and center tunnel. however, it is impracticable to connect a honeycomb panel to other members by simple spot welding because it includes a honeycomb core which is held between an upper and a lower plate and full of bores. The honeycomb core is often implemented with paper for the purpose of reducing the cost and therefore cannot be simply connected to other members by bolts and nuts. Should holes be formed through the honeycomb core, water would be admitted into the core through those holes to wet and break the core. In the aircraft art the holes for bolts are protected by special grommets as disclosed, for example, in U.S. Pat. Nos. 4,399,642 and 4,296,586. Such grommets, however, are not feasible for motor vehicles and the like which are produced on a quantity basis because they not only add to the cost but also permits no machining errors.
Furthermore, in the case that the floor panel is configured independently of the center tunnel and connected to the center tunnel as well as to the side sills, an existing assembly line for vehicle bodies has to be modified to a significant extent.